The technology of magnetic refrigeration at room temperature is known for more than twenty years and we know its advantages in terms of ecology and sustainable development. We also know its limitations in effective heat capacity and thermal efficiency. Therefore, research in this field tends to improve the performance of such a generator by acting on various parameters like the strength of the magnetic field, the performances of the magnetocaloric materials, the heat exchange surface between the heat transfer fluid and the magnetocaloric materials, the performance of the heat exchangers, etc.
The choice of the magnetocaloric materials is a determining factor and has a direct impact on the performances of a magnetocaloric thermal generator. The magnetocaloric effect reaches a peak around the Curie temperature of magnetocaloric materials. It is known to associate many magnetocaloric materials with different Curie temperatures in order to operate a magnetocaloric thermal generator over a wide range of temperatures.
Thus, many magnetocaloric thermal generators use the magnetocaloric effect of several magnetocaloric materials by circulating a heat transfer fluid along or through said magnetocaloric materials, in two opposite directions according to phases of increase or decrease of the magnetic field to which the magnetocaloric materials are subjected. At the start up of such a thermal generator, the circulation of the fluid allows to obtain a temperature gradient between both ends of the magnetocaloric material. The achieving of this temperature gradient depends on several factors like the initial temperature, the flow rate of the heat transfer fluid, the intensity of the magnetocaloric effect, the Curie temperature and the length of the magnetocaloric materials. The more the initial temperature and the Curie temperature of the magnetocaloric materials are close, the more a temperature gradient, with which the generator is functional and can produce or exchange thermal energy with an external circuit, will be rapidly reached. Now, the initial temperature of the heat transfer fluid and of the magnetocaloric materials is not controlled and is equal to the outside temperature of the generator. This temperature can be comprised in a very wide range of temperatures, for example between −20 and +60° C. This implies that to achieve the temperature gradient, i.e. the operational phase of a thermal magnetocaloric generator, it can take a long time.
Moreover, the operation over a wide range of temperatures implies that the magnetic system which is generally constituted by an assembly of permanent magnets is subjected to an important temperature variation. Indeed, the magnetocaloric materials are generally disposed in the magnetic gap of the magnet assembly and lead a temperature change of the magnetic system through thermal convection. For that purpose, FIGS. 1A and 1B illustrate a thermal generator comprising a magnetic system with two magnets M1 and M2 creating a gap G in which two magnetocaloric materials MC1 and MC2 move. Almost the whole volume of the gap is alternately filled with one magnetocaloric material MC1 or MC2. When one of said magnetocaloric materials MC1 and MC2 is in the gap, there is a minimal space between the magnets M1, M2 and this magnetocaloric material MC1, MC2, in order to increase the magnetic effect and thus the thermal power. The first magnetocaloric material MC1 has a Curie temperature of 0° C. and an operating or transition area comprised between −10° C. and +10° C. and the second magnetocaloric material MC2 has a Curie temperature of 20° C. and an operating or transition area comprised between +10° C. and +30° C. FIG. 1A shows a first phase of the cycle in which the first magnetocaloric material MC1 is subjected to an increasing magnetic field and the second magnetocaloric material MC2 is subjected to a decreasing magnetic field and FIG. 1B shows the second phase of the cycle in which the first magnetocaloric material MC1 is subjected to a decreasing magnetic field and the second magnetocaloric material MC2 is subjected to an increasing magnetic field. The thermal amplitude undergone by the magnets is about 40° C. (from −10° C. to +30° C.). The magnets, with their thermal inertia, have a negative impact on the temperature gradient in the magnetocaloric materials MC1 and MC2: they thermally exchange with said magnetocaloric materials MC1 and MC2, which reduces the temperature gradient of the magnetocaloric materials. The result is that the performance of such a thermal generator, which is bound to this temperature gradient, is reduced.